Add a smooth streaming segmenter muxer

This muxer splits the output from the ismv muxer into individual
files, in realtime.

The same can also be done by the standalone tool ismindex, but this
muxer is needed for doing it in realtime (especially for live
streams that need extra handling for updating the lookahead fields
in the fragment headers).

Using this muxer, one can deliver live smooth streaming from a
normal static file web server. (Using ismindex, one can deliver
premade smooth streaming files from a static file web server,
or prepare files for serving with IIS.)

Signed-off-by: Martin Storsjö <martin@martin.st>